Chopped glass fiber strands are commonly used as reinforcement materials in thermoplastic articles. It is known in the art that glass fiber reinforced polymer composites possess higher mechanical properties compared to unreinforced polymer composites, provided that the reinforcement fiber surface is suitably modified by size chemical formulation. Thus, better dimensional stability, tensile strength and modulus, flexural strength and modulus, and impact resistance and creep resistance can be achieved with glass fiber reinforced composites.
Typically, the glass fibers are formed by drawing molten glass into filaments through a bushing or orifice plate, applying a sizing composition containing lubricants, coupling agents and film-forming binder resins to the filaments, gathering the filaments into strands, chopping the glass fiber strands into segments of the desired length, and drying the sizing composition. These chopped fiber strand segments are thereafter mixed with a polymeric resin, and the mixture supplied to a compression- or injection-molding machine to be formed into glass fiber reinforced plastic articles. Typically, the chopped fiber strands are mixed with pellets of a thermoplastic polymer resin, and the mixture supplied to an extruder wherein the resin is melted, the integrity of the glass fiber strands is destroyed, the fiber strands are dispersed throughout the molten resin, and the fiber strand/resin dispersion is formed into pellets. These pellets are then fed to the molding machine and formed into molded composite articles having a substantially homogeneous dispersion of the glass fiber strands throughout.
Unfortunately, however, chopped glass fiber strands made via such processes are typically bulky and do not flow well. Consequently, such chopped strands are difficult to handle and have been problematic in automated processing equipment.
One attempt at solving this problem has been to compact the chopped fiber strands into denser rod-shaped bundles or pellets to improve their flowability, and to enable the use of automated equipment to weigh and transport these pellets for mixing with the thermoplastic polymer resins. Such a process is disclosed in U.S. Pat. No. 4,840,755, wherein wet chopped fiber strands are rolled, preferably on a vibrating carrier, to round the strands and compact them into denser, cylindrically shaped pellets. While such methods and apparatus tend to provide denser, more cylindrically shaped pellets exhibiting better flowability, they are undesirably limited in certain respects. For example, in such pellet-forming processes, the pellet size and fiber content are generally limited by the size and number of fibers in the chopped strand, because the process is designed to avoid multiple chopped strand segments from adhering together to form pellets containing more fibers than are present in a single chopped strand. Consequently, to obtain pellets having a suitable bulk density and a sufficient ratio of diameter to length to exhibit good flowability, the fiber strand from which the segments are chopped usually must be formed of a large number of filaments. However, increasing the number of filaments required to be formed and combined into a single strand undesirably complicates the forming operation.
In an attempt to overcome these shortcomings, U.S. Pat. No. 5,578,535, which is herein incorporated by reference in its entirety, discloses glass fiber pellets that are from about 20 to 30 percent denser than the individual glass strands from which they are made, and from about 5 to 15 times larger in diameter. These pellets are prepared by (i) hydrating cut fiber strand segments to a level sufficient to prevent separation of the fiber strand segments into individual filaments but insufficient to cause the fiber strand segments to agglomerate into a clump; and (ii) mixing the hydrated strand segments by a suitable method for a time sufficient to form pellets. Suitable mixing methods include processes that keep the fibers moving over and around one another, such as tumbling, agitating, blending, commingling, stirring and intermingling. Although these pellets can be made by such diverse mixing methods, it has been discovered that many of these methods are either too inefficient to be used commercially, or cannot be adequately controlled to produce sufficiently uniform pellets to provide the composite article made therefrom with the strength characteristics comparable to a composite article made from non-pelleted chopped fiber strands. For example, the use of a modified disk pelletizer frequently results in excessive residence time of the formed pellets within the mixer, which causes the pellets to rub against each other for an excessive period, which in turn results in degradation of the pellets, due to their abrasive nature. Such pellet degradation ultimately reduces the strength characteristics of the molded composite articles.
Another problem commonly known to pellets made from fiber strands that are made for use as reinforcements in composites and other fiber-reinforced products is the discoloration they could cause to the thermoplastic during compounding and/or heat ageing of the molded part. This discoloration is typically seen as an undesirable yellowing of the thermoplastic that is thought to be related to some of the materials used to size the fiber strands, including, but not limited to, the binders and film formers used in the sizing compositions used to treat the fiber strands.
Discoloration in molded composite products, or in the materials used to manufacture molded composite products, may arise from the presence of contaminants in one or more materials that make up the composite formulation, or from the presence of impurities in the ingredients that are used to form fiber-reinforced composites. These ingredients may be materials used in sizing compositions for coating reinforcing fibers before they are molded into composites. For example, conventional sizing compositions often impart a yellow color or other discoloration to the compounds when such sizings are applied. These discolorations may be carried into the composite product when the reinforcements are molded. These discolorations may be caused by oxidatative decomposition of unsaturated chemicals, such as fatty unsaturated surfactants and/or lubricants, which are of low thermal stability. These discolorations may also be caused by nitrogen containing compounds, such as amides, imides, cationic surfactants or amine-based chemicals, which are used, for example, as neutralizing agents.
Historically, the problem of discoloration has been partially addressed by adding ingredients to the composite formulation to counteract the discoloration before the composite formulation is molded. Frequently, antioxidants are used in the compounding formulations to minimize thermal degradation and associated discoloration. Also, the added ingredient may be a colorant, e.g., pigment or dye, that changes the color of the composite formulation. For example, a blue pigment or dye may be added to the composite formulation to combat a yellow discoloration and, as a result, the finished molded composite appears whiter.
A more recently developed method of correcting discoloration has been adapted to fiber-reinforced composite manufacturing. Although it has traditionally been used in compositions applied to paper products, clothing, and plastics to create a brilliant white appearance, the method involves adding an optical brightener, such as a fluorescent whitening or brightening agent, to the composite formulation or to the sizing compositions that are applied to the fiber reinforcements used to mold composites. U.S. Pat. No. 5,646,207, for example, describes a sizing composition that includes a fluorescent whitening agent in addition to other sizing ingredients such as a carboxylated polypropylene, a silane coupling agent, and a lubricant. However, compositions such as those disclosed in this patent rely specifically on the presence of the fluorescent whitening agent to reduce discoloration in the composite product.
Use of an optical brightener does not, however, satisfactorily solve the problem of discoloration of the molded composite. According to U.S. Pat. No. 5,646,207, discoloration problems in the molded composite remain when the fluorescent whitening agent is added to the composite formulation because, in order to prevent discoloration satisfactorily, the fluorescent whitening agent must be well dispersed into the matrix polymer of the composite formulation. At the same time, this patent notes that uniform dispersion of the fluorescent brightener in the matrix resin is difficult to achieve.
Other technical and economic problems stem from the use of optical brighteners such as a fluorescent whitening agent in composite formulations and in particular, in sizing compositions for fiber reinforcements. Technical problems may compromise the quality of the composite matrix polymer or undesirable interactions with other composite ingredients. For example, an optical brightener typically accelerates degradation of the matrix polymer when it is exposed to ultraviolet (UV) light or other forms of radiant energy. Moreover, optical brighteners themselves can degrade chemically over time, and thus contribute to yellowing or other discoloration of the molded composite articles. Another observed problem arises when an optical brightener reacts with other ingredients such as an antioxidant that may be added to the composite formulation. In this regard, combining the optical brightener and the antioxidant reduces the efficiency of both ingredients, and ultimately results in discoloration of the composite.
Additionally, it has been observed that color matching of composite batches is difficult to achieve when the composite contains optical brighteners. In order to compensate for these difficulties in color matching, in some cases varying amounts of pigments or other additives have been added to the composite, further complicating the effort to maintain consistent color between batches. The difficulties encountered in turning out composite batches having consistent color, in turn, increases the cost of production by requiring more starting materials and higher labor costs, and therefore poses an economic disadvantage in addition to the technical problems. Further, color analysis of molded articles that contain optical brighteners is difficult because the articles behave differently under different lighting types and conditions. These problems with color analysis also increase the costs of producing the fiber reinforcements and/or the composite product. The use of optical brighteners further contributes to increased production costs simply because they tend to be relatively expensive chemicals.
In some applications, it may be desired that the molded composite product have a white color. In this regard, whitening pigments have been added directly to the composite molding composition to provide white coloration. One such typically used whitening pigment is powdered titanium dioxide (TiO2). However, the addition of whitening pigments such as TiO2, which may also act as abrasives, tends to result in damage to the reinforcing glass fibers and dramatically reduce the mechanical strength of the composite.
Therefore, there is a need in the art for a cost-effective sizing composition for use in preparing chopped strands and glass fiber pellets which have little or no coloration and provide increased whiteness, brightness, color stability and/or color compatibility in a molded composite product without requiring the use of an optical brightener or whitener.